1. Field of the Invention
The present invention relates to an image forming apparatus, and more particularly to an image forming apparatus having a serial head fitted with a sub tank which connects to an ink supply main tank.
2. Description of the Related Art
Among image forming apparatuses using a serial head, which moves reciprocally in a direction perpendicular to the conveyance direction of paper while ejecting ink toward the paper, there are image forming apparatuses that perform printing by scanning the paper with the serial head that is integrated with an ink supply sub tank having the capacity limited to the minimum necessary capacity. In an image forming apparatus of this kind, the sub tank is connected through an intake port to a main tank, which is provided separately, when the ink is supplied from the main tank to the sub tank.
An image forming apparatus of this kind is disclosed in Japanese Patent Application Publication Nos. 2002-086747 and 2002-240323. FIG. 9 is an approximate oblique drawing of a sub tank 101, which forms a liquid storage vessel disclosed in Japanese Patent Application Publication No. 2002-086747. As shown in FIG. 9, the sub tank 101 is provided with an ink supply needle 102 of an ink intake port, an ink supply channel 103, a lid member 104, a gas/liquid separating member 105 having fine apertures 107, an air intake port 106, and the like.
The sub tank 101 having a composition of this kind has the following function. When supplying ink, ink is supplied from the needle 102 by means of a negative suction pressure created by a pump, or the like, connected to the air intake port 106. The suction pressure of the pump is adjusted in such a manner that the ink supply is halted by the action of the non-wetting material coating the gas/liquid separating member 105 and the meniscus force created in the fine apertures 107 when the ink supply progresses and the position of the surface (the interface with the atmosphere) of the ink reaches the gas/liquid separating member 105 at the upper surface of the sub tank 101. Thereupon, as ink is ejected during printing, the ink inside the sub tank 101 is gradually consumed, and the ink level gradually declines. At a suitable timing, ink is then supplied through the needle 102.
In Japanese Patent Application Publication No. 2002-086747, it is explained that decline in the functional performance of the gas/liquid separating member 105 is prevented and the durability of this member is improved, due to the action of the non-wetting material coating the gas/liquid separating member 105.
Japanese Patent Application Publication No. 2002-240323 discloses a sub tank which is similar to that of Japanese Patent Application Publication No. 2002-086747. In Japanese Patent Application Publication No. 2002-240323, it is explained that the concentration of surfactant in ink is adjusted so as to prevent the ink from entering into the fine apertures of the gas/liquid separating member, and decline in the functional performance of the gas/liquid separating member is thereby prevented and the durability of this member is thus improved.
In Japanese Patent Application Publication Nos. 2002-086747 and 2002-240323, it is explained that the durability of the gas/liquid separating member 105 is improved and the lifespan of the gas/liquid separating member 105 before replacement is increased; however, the gas/liquid separating member 105 still has a short lifespan compared to that of the printer, and still needs to be replaced periodically. Hence, if the gas/liquid separating member 105 is not replaced at the time that such replacement has become necessary, then the ink enters into the fine apertures 107, and the like, of the gas/liquid separating member 105, thereby forming meniscuses, and the fine apertures 107 may become sealed off due to the effects of the surfactant contained in the ink. Consequently, there is a risk that the functional performance of the gas/liquid separating member 105 may decline, making it more difficult to supply ink to the sub tank, among other problems.
Moreover, functional decline in the gas/liquid separating member 105 is difficult to recognize from external observation of the image forming apparatus, and it is difficult for the user to judge whether or not the time for replacement of the gas/liquid separating member 105 has been reached.